Devices have already been proposed in the art—e. g. U.S. Pat. No. 6,763,715—including locking-unlocking devices on a rotating mandrel of balancing machines, which foresee the use of a screw pressure ring nut which is inserted on the mandrel and unscrewed and removed from it for every wheel to be balanced, as well traction springs with short work stroke. This is a damaging need for the functionality of the balancing machine. Moreover, both the pressure ring nut and the springs must be driven into rotation with the mandrel, which is a source of balancing inaccuracies due to imbalances created during the rotation by the springs, which are subjected to oscillations during their rotation. If one must work with wheels structured in a special manner, it is necessary to support them for their correct locking on the mandrel, which is tiring for the operator.